This invention pertains to refining of petroleum and, more particularly, to catalytic cracking of oil.
Catalytic cracking of oil is an important refinery process which is used to produce gasoline and other hydrocarbons. During catalytic cracking, the feedstock, which is generally a cut or fraction of crude oil, is cracked in a reactor under catalytic cracking temperatures and pressures in the presence of a catalytic to produce more valuable, lower molecular weight hydrocarbons. Gas oil is usually used as the feedstock in catalytic cracking. Gas oil feedstocks typically contain from 55% to 80% gas oil by volume having a boiling range from 650.degree. F. to 1000.degree. F. and less than 1% RAMS carbon by weight. Gas oil feedstocks also typically contain less than 5% by volume naphtha and lighter hydrocarbons having a boiling temperature below 430.degree. F., from 10% to 30% by volume diesel and kerosene having a boiling range from 430.degree. F. to 650.degree. F., and less than 10% by volume resid having a boiling temperature above 1000.degree. F.
In conventional catalytic cracking, whole crude oil is separated in a primary pipestill (crude oil unit) or atmospheric tower into fractions of 200.degree. F. and lighter material, naphtha, diesel oil, atmospheric gas oil, and atmospheric bottoms. The atmospheric bottoms are heated in a furnace and separated in a secondary pipestill or vacuum tower into fractions of vacuum naphtha, light vacuum gas oil, heavy vacuum gas oil, and resid. The atmospheric gas oil from the atmospheric tower and the light and heavy gas oils from the vacuum tower and subsequently pumped into the catalytic cracker as a blended composite gas oil feedstock, where it is contacted with fine solid catalyst particles under cracking conditions to crack the gas oil. During cracking, the catalyst becomes coked and deactivated and has to be regenerated in a regeneration vessel. Fresh catalyst is conventionally replaced in the catalytic cracker at a rate of 0.25 pounds per barrel of reactor feed.
Catalytic cracking is an important source of gasoline. From time to time, however, it is necessary to shut down the catalytic cracking unit for days, weeks, or even months to clean, unplug, maintain, uncoke, revamp, and/or repair the pipestill (crude unit) vacuum tower and/or atmospheric tower. When the crude unit is down for maintenance or repair, there is no gas oil feed for the catalytic cracking unit. The catalytic cracking unit would normally be shutdown if gas oil cannot be obtained from another source. Such shutdown deprives the refinery and the consumer of substantial amounts of gasoline. It is also very expensive. Revamp costs and revenue loss to the refinery during shutdown can add up to millions of dollars. Shutdown of the catalytic cracker was, heretofore, required when the pipestills (crude unit), vacuum tower, and/or atmospheric tower were taken offstream for maintenance, revamp, or other work since there was no longer any production of gas oil feedstock from the pipestills (crude unit) and towers. It was generally believed that the catalytic cracking unit could not be operated nor the required heat balance maintained when using unrefined whole crude oil as the feedstock.
Typifying some to the many prior art catalytic cracking units, regenerators, and other refinery equipment and processes are those shown in U.S. Patents: 2,382,382; 2,398,739; 2,398,759; 2,414,002; 2,425,849; 2,436,927; 2,884,303; 2,981,676; 2,985,584; 3,004,926; 3,039,953; 3,351,548; 3,364,136; 3,513,087; 3,563,911; 3,661,800; 3,838,036; 3,844,973; 3,909,392; 4,331,533; 4,332,674; 4,341,623; 4,341,660; 4,332,674; and 4,695,370.
It is, therefore, desirable to provide an improved catalytic cracking process which is operable when the upstream pipestills (crude unit) or towers are taken off line for revamp, maintenance, or to shutdown permanently to consolidate operations.